An X-ray generator in general is a device which irradiates a thermoelectron beam flux from a cathode to an anode, and generates an X-ray flux at the anode. The X-ray flux generated by the X-ray generator is irradiated on a subject, and the X-ray flux transmitted through the subject is detected by a predetermined detection means, to thereby obtain information of a part through which the X-ray flux has transmitted, as a slice image. A tomograph is a device which irradiates an X-ray flux from an X-ray generator on a subject, detects the X-ray flux transmitted through the subject by detection means arranged in a plane, and obtains slice data based on the detected information, which data is used for diagnosing a state of the slice.
As one of the most typical devices using an X-ray irradiator, an X-ray CT scanner (hereinbelow, referred to as “CT (Computed Tomography)”) has been known. The CT is, for example, composed of an X-ray irradiator and a detection means which is a combination of a scintillator and a semiconductor device. However, for the CT, any device can be used in combination with the X-ray irradiator, as long as the device can detect an X-ray. The X-ray CT scanner is an X-ray diagnostic apparatus in which a large number of detectors each composed of a combination of the scintillator and the semiconductor device revolve around a body axis of a subject, while the X-ray irradiator and the detectors keep a flanking position relative to the subject, and slice images in a body width direction are continuously obtained in a body axis direction.
It should be noted that the body axis direction and the body width direction are orthogonal to each other, and an irradiation axis of the X-ray flux is orthogonal to both the body axis direction and the body width direction.
The CT is provided with a gantry and a platform. The gantry is formed in a shape of a thin-walled cylinder, and a central axis in a hollow part of the gantry is oriented so as to be included in a plane (hereinbelow, referred to as “axial plane”) containing an incoming axis of the thermoelectron ray and an irradiation axis of the X-ray flux. The platform is placed to be movable backward and forward in the hollow part of the gantry in such a manner that the body axis of the subject to be subjected to the X-ray flux irradiation coincides with the central axis of the gantry. Further, the gantry has an X-ray tube and detectors at an opposed position to a position of the X-ray tube across the hollow part. As the X-ray tube and the detectors revolve around the body axis of the subject lying on the platform in the hollow part, the platform moves forward/backward. During this movement, the X-ray flux irradiated from the X-ray tube and transmitted through the subject is detected by the detectors to thereby obtain slice data along a predetermined length of the subject in the body axis direction. The slice data are subjected to computer analysis and a number of slice image data are created, which are then used for diagnosis of an inside of the subject.
Accordingly, based on differences in intensity of the incoming X-ray flux that has transmitted through the subject into respective detectors, a state of a slice part through which the X-ray has transmitted can be determined. In order to reduce exposure of the subject, it is preferred that an X-ray intensity angular distribution of the X-ray flux be made to fall in the lowest range that can be detected by the detectors, and at the same time, difference range of the X-ray flux intensity be made as small as possible.
The term “slice data” used herein means electrically generated data of a state of a slice part of the subject, based on the differences in intensity of the incoming X-ray flux that has transmitted through the subject into respective detectors.
The term “image data” used herein means data visually represented as an image based on the slice data.
In order to reduce the exposure of the subject as much as possible, the CT has the following means. For example, in a case where the subject is a human being (hereinbelow, frequently referred to as “examinee”), when the examinee at the irradiation position is seen in the body axis direction, a center of the examinee is the thickest and both ends in the body width direction are the thinnest. Therefore, during the X-ray flux irradiation, an absorption amount of the X-ray by the examinee body becomes largest at the central part of the body, and becomes smaller towards the both ends of the body. Attempts have been made to adjust such a difference in absorption of an X-ray flux in the body width direction due to the thickness of the examinee to fall in the lowest range that can be detected by the detectors (hereinbelow, referred to as “appropriate range”), by disposing a wedge filter between the X-ray tube and the examinee. The wedge filter is made of aluminum or the like, and a transmissive surface thereof is a cylindrical concave surface formed in such a manner that a vertical section seen in the body axis direction is in a shape of a concave lens which is axisymmetrical relative to the irradiation axis. With this configuration, a relatively strong X-ray that has transmitted through a central thinner part of the cylindrical concave wedge filter reaches the central part of the body, while a relatively weak X-ray that has transmitted through thicker lateral parts of the cylindrical concave wedge filter reaches thinner parts of the body. In this manner, the intensity of the X-ray flux is adjusted by the difference in the filter thickness so as to correspond the difference in body thickness while the X-ray flux transmits through the wedge filter.
It should be noted that, since the wedge filter revolves uniformly with the x-ray detector and the detectors, the filter in practice is designed based on a cross section of the examinee as a perfect circle.
On the other hand, in a case of X-ray flux irradiation in the body axis direction of the examinee, when the X-ray flux is irradiated by the X-ray generator, a phenomenon called “heel effect” occurs, in which an X-ray intensity angular distribution on an axis orthogonal to the irradiation axis at a predetermined distance from the anode becomes a shape with a cone angle (a shape of an approximate sector), on the axial plane containing the beam irradiation axis of the thermoelectron beam flux and the irradiation axis of the X-ray flux. Due to this heel effect, when the X-ray flux is irradiated on the examinee, the X-ray intensity angular distribution in the body axis direction becomes nonuniform. In other words, when obtaining slice data along the body axis, the thickness of the examinee body is considered to be even in the body axis direction, and an irradiation amount in this direction is kept uniform. As a result, at a portion with a strong X-ray irradiation intensity, the irradiation amount becomes excessively high, and the body part of the examinee is overexposed.
However, the overexposure due to this heel effect has not been taken into account, and a part of the slice data obtained by detecting the transmitted X-ray flux by the detector becomes blurry. For obtaining clear slice data, the heel effect has been cancelled merely by adjusting the obtained slice data themselves. For example, a proposal has been made in which an intensity distribution of various parts in the irradiation space of the X-ray flux is measured in advance by a sensor and the like without the examinee on the platform; data prepared in advance are referred to every time X-ray image is obtained; and the intensity distribution is adjusted by a computer program so that variation in the intensity distribution is cancelled (see, for example, Patent Document 1).
Recently, there has been disclosed an X-ray irradiator which uniformly irradiates X-ray by use of a metallic filter (see, for example, Patent Document 2).
Specifically, the invention disclosed in Patent Document 2 provides an X-ray irradiator having: an X-ray tube which outputs X-ray generated by irradiation of an electron beam on a target from an X-ray irradiation opening to outside; and a metallic filter which is attached to the X-ray irradiation opening of the X-ray tube and configured in such a manner that a portion of the filter exposed to a larger irradiation dosage is made thicker, based on the measurement of an X-ray dose distribution output from the X-ray irradiation opening of the X-ray tube.
In addition, there is a problem of artifacts (obstructive shadow) which are always present in X-ray CT imaging. An artifact is a phenomenon in which a virtual image is included in slice image data due to various factors, such as a failure of a device, defects in an image reconstruction system and scanning conditions. For example, it is considered that a ring-shaped artifact results from a failure of a detector, and that a beam hardening artifact results from a difference in energy of an outgoing X-ray due to energy absorption during transmission of an X-ray flux through a subject. Generation of artifacts lowers accuracy of diagnosis or the like on a subject based on slice image data.
In order to reduce generation of the artifacts, attempts has been made in which an image is first obtained and a cause is specified from a type and shape of the artifact and then removed; or in which image data are adjusted by a computer program (see, for example, Non-patent Document 1).
(Patent Document 1)
Japanese Unexamined Patent Publication Kokai No. 2000-079114 (paragraphs 0008-0029 and FIG. 2)
(Patent Document 2)
Japanese Unexamined Patent Publication Kokai No. 2004-214130 (claim 1)
(Non-Patent Document 1)
Katsumi Tsujioka, “Mechanical Engineering of X-ray CT scanner (5)-Artifact-” (PDF file), p 737, 6. Artifact Due to Cone-Angle of Multi-Slice CT, (online), Fujita Health University School of Health Sciences, (searched on Mar. 16, 2005), internet (URL: http://www.fujita-hu.ac.jp/˜tsujioka/education.html)